Electrostatic traps (ESTs) are a class of ion optical devices where moving ions experience multiple reflections in substantially electrostatic fields. Unlike in RF fields, trapping in electrostatic traps is possible only for moving ions. To ensure this movement takes place and also to maintain conservation of energy, a high vacuum is required so that the loss of ion energy over a data acquisition time Tm is negligible.
There are three main classes of EST: linear, where ions change their direction of motion along one of the coordinates of the trap; circular, where ions experience multiple deflections without turning points; and orbital, where both types of motion are present. The so-called Orbitrap mass analyser is a specific type of EST that falls into the latter category of ESTs identified above. The Orbitrap is described in detail in U.S. Pat. No. 5,886,346. Briefly, ions from an ion source are injected into a measurement cavity defined between inner and outer shaped electrodes. The outer electrode is split into two parts by a circumferential gap which allows ion injection into the measurement cavity. As bunches of trapped ions pass a detector (which, in the preferred embodiment is formed by one of the two outer electrode parts), they induce an image current in that detector which is amplified.
The inner and outer shaped electrodes, when energized, produce a hyper-logarithmic field in the cavity to allow trapping of injected ions using an electrostatic field. The potential distribution U(r,z) of the hyper-logarithmic field is of the form
                              U          ⁡                      (                          r              ,              z                        )                          =                                            k              2                        ⁡                          [                                                z                  2                                -                                                      r                    2                                    2                                            ]                                +                                    k              2                        ⁢                                                            (                                      R                    m                                    )                                2                            ·                              ln                ⁡                                  [                                      r                                          R                      m                                                        ]                                                              +          C                                    (        1        )            where r and z are cylindrical coordinates and z=0 is the plane of symmetry of the field) C is a constant, k is the field curvature and Rm>0 is the characteristic radius.
In this field, the motion of ions with mass m and charge q along the axis z is described as a simple harmonic oscillator with an exact solution for q,k>0:
                                          z            ⁡                          (              t              )                                =                                    A              z                        ·                          cos              ⁡                              (                                                                            ω                      0                                        ⁢                    t                                    +                  θ                                )                                                    ⁢                                  ⁢        where                            (        2        )                                          ω          0                =                              qk            m                                              (        3        )            and T0 thus defines the frequency of axial oscillations in radians per second, and Az and 2 are the amplitude and phase of axial oscillations, respectively.
Whilst the foregoing discusses the theoretical situation, in which the electrodes are of ideal hyper-logarithmic shape, in reality there is a limit to the accuracy with which any practical construction can approximate that ideal geometry. As discussed in “Interfacing the Orbitrap Mass Analyser to an Electrospray Ion Source”, by Hardman et al, Analytical Chemistry Vo. 75, No. 7, April 2003, any divergence from the ideal electrode geometry, and/or inclusion of electrical perturbations, will result in a perturbation to the ideal field which in turn will transform the harmonic axial oscillations of the ideal field into non-linear oscillations. This in turn may result in a reduction in mass accuracy, peak shape and height, and so forth.
The present invention, in general terms, seeks to address problems arising from the non-ideal nature of a real electrostatic trap.